Technical Field
The invention relates to a method for the controlled operation of a heated industrial furnace, in particular a regeneratively heated industrial furnace, and to an open-loop and closed-loop control unit and to a heatable industrial furnace.
Description of the Related Art
In principle, an industrial furnace of the type mentioned at the beginning is not restricted to use in glass production. For example, an industrial furnace of the type mentioned at the beginning can also be used in metal production or the like. A regenerative industrial furnace of the type mentioned at the beginning has however proven to be particularly suitable in glass production for the melting of glass.
In the case of a heated industrial furnace, in particular a regeneratively heated industrial furnace, there is often a separate supply of fuel and of a gaseous oxygen carrier, in particular combustion air and/or oxygen, so that for this reason alone a special open-loop and closed-loop control is required, as distinct from burner controls, which supply combustion air and/or oxygen by way of a common burner.
Until now, the control of regenerative glass melting furnaces—i.e., often by means of control by way of the upper furnace in the furnace chamber as a controlled system—has been exclusively entrusted to closed-loop controllers, such as PID controllers or the like, which are aimed at controlling the furnace temperature, specifically generally the upper furnace temperature, and the output of which either represents a quantity of fuel itself or a quantity of combustion air, which however the quantity of fuel then follows in an adjustable relationship; there is therefore always a quantity of fuel at the output of such control systems.
According to generally adopted practice, a charge of fuel gas as a fuel to industrial furnaces is continuously corrected, either manually by the operator of the installation or by an automatic controller, in order to counteract detectable changes of the measured furnace temperature and maintain the setpoint temperature as constantly as possible. If variations of the energy content of the fuel gas thereby occur, it is expected that the operator of the installation or the temperature controller will likewise compensate for this—by then choosing the necessary quantity of fuel gas—in such a way that the specified setpoint temperature can be maintained.
On account of the high heat-retaining capacity of an industrial furnace, including the material it is heating, however, changes in temperature usually only become apparent after hours, and their correction takes a similarly great or longer amount of time, because the adapted quantity of fuel gas first has to be found by the control algorithm. A particularly advantageous control method of the applicant's, on the basis of a temperature control, is known for example from DE 10 2010 041 155 A1. It is explained therein that, among the prerequisites for efficient heating are a stable, uniform flow of fuel without unnecessary variations, and possibly the thermal symmetry of regenerators.
When there is an existing calorific value measurement or other on-line measurement of the energy content of the fuel gas, it is usual to adapt the setpoint value of the quantity of fuel gas manually or automatically in accordance with a measured change in the calorific value or the Wobbe index of the fuel gas. However, it is widely the case that the air ratio between the fuel gas and the combustion air is not changed thereby.
DD 213 746 describes a method for optimizing gas-air mixtures, in particular for town gas, in which therefore the fuel-air mixture is changed in dependence on the Wobbe index of the fuel gas and the O2 content in the flue gas. For this purpose there takes place the feedforwarding of a measured change in the Wobbe index to the control of the fuel gas supply, and at the same time the feedforwarding of the volumetric flow of fuel, the Wobbe index and a measured O2 content as a correction of the setpoint value of the fuel-air supply. However, experience shows that, in particular for near-stoichiometric combustion, the introduction of a controlling correction, first of the quantity of fuel gas and then once again of the quantity of combustion air, leads to an undesired inaccuracy of the control, which for the method thus described is evidently corrected again subsequently by the additional feedforwarding of a measured O2 concentration in the flue gas. For the increasingly more important case of near-stoichiometric or deliberately substoichiometric combustion, this method is no longer suitable.
To an increasing extent, industrial furnaces are thus being supplied with fuel gases of varying composition, since the fuel gases are increasingly frequently being mixed from different sources, for example natural gas from Russia, natural gas from the North Sea, natural gas from North Africa, natural gas from fracking, etc.
Variations of the fuel gas composition have concomitant effects:                on the flow of energy that is input into the industrial furnace,        on the associated requirement for combustion air or oxygen,        on the correctness of the volumetric fuel gas flow measurement as a consequence of varying gas density,        on the thermal efficiency of the flames as a result of changed heat losses with the flue gas flow emerging from the furnace chamber,        on the combustion dynamics of the flames in the industrial furnace in response to the flame pulse.        
However, the conventional control methods thus prove to be too slow. In particular, they do not ensure the predictive reaction to measurable fuel gas variations, i.e., they only react to a changing of furnace temperatures that becomes evident very much later, for example possibly hours later.
Allowance should also be made for the fact that the fuel charge and the dosing of oxygen or combustion air must be corrected at the same time in order to ensure firing that remains constant. Industrial furnaces are all the more sensitive to such problems in this respect the closer the combustion is operated to near-stoichiometric, for example in order to keep down the emission of nitrogen oxide and similarly the energy consumption.
Allowance should also be made for the fact that the on-line measurement of the energy content of a fuel gas and the simultaneous on-line determination of the stoichiometric combustion air/oxygen requirement per unit of energy impose demanding requirements on the measuring instrumentation required for this purpose and its accuracy (gas chromatographs); for relatively small and medium-sized installations, meeting these requirements often involves a disproportionate effort.
Accordingly, a control method that can dependably provide the metering both of fuel gas and of oxygen and/or combustion air that is suitable for ensuring constant and optimized combustion conditions just by a reliable on-line measurement of the calorific value of the fuel gas would be desirable. A particularly advantageous control method of the applicant's, in particular also for the increasingly more important case of near-stoichiometric or deliberately substoichiometric combustion, is known for example from DE 10 2010 041 157 A1. The stated control method has a lambda control with infiltrated air indication, which is used to compensate for varying infiltrated air. It has also been recognized there that a regeneratively fired industrial furnace can be observed to undergo typical recurring trend patterns of uncontrolled infiltrated air ingress or uncontrolled infiltrated air loss that cannot be compensated, or only incompletely, by a gradual and slow correction of the air ratio, while a rapid correction of the air ratio fails on account of unfavorable control dynamics of the industrial furnace.
If, during its application, variations of the fuel gas composition occur, this is interpreted as a variation of the infiltrated air and is corrected. Although this approach is heading in the right direction, the mixing of different physical effects (for example changes of the actual infiltrated air and changes of this stoichiometric combustion air/oxygen requirement per unit of energy as a consequence of variations of the fuel gas composition) easily leads to problems in the accuracy or dynamics of the control.
Accordingly, a control method that compensates for the influence of variations of the actual infiltrated air and the influence of variations of the fuel gas composition in different controlled variables, but acting at the same time effect, would also be desirable. Such a control method should be as robust as possible and as insensitive as possible to variations in accuracy.